1. Field of the Invention
This invention relates to a core of a solenoid actuator for electromagnetically driving a driven member, and more particularly to a multilayer core formed by a laminated stack of a plurality of magnetic plates.
2. Description of the Prior Art
Conventionally, a solenoid actuator of this kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 11-273945, which is applied to a valve-actuating mechanism for opening/closing a valve (gas exchange valve) of an internal combustion engine, and includes an armature and upper and lower electromagnets for vertically attracting the armature.
Each of the electromagnets includes a core having an E shape in cross section. The recessed portions of the E-shaped core serve as a coil groove opposed to the armature and accommodating a coil. The core is a unitary assembly formed of a center core member and a multiplicity of laminates stacked on opposite sides of the center core member. The center core member is formed of silicon steel which is larger in thickness than each laminate and has an E shape in side view. The laminates are each formed of a composite magnetically soft material having the same shape and size as those of the side face of the center core member and are stacked on the opposite sides of the center core member as described above. The center core member and the multiplicity of laminates are welded together to form the unitary component, and the end faces of the center core member and the laminates opposed to the armature form a flat attracting surface for receiving the magnetically attracted armature thereat. The reason why the core (laminated core) having the multilayer construction described above is employed is that it is possible to reduce core loss during energization of the electromagnet compared with the case of a solid core is used, thereby ensuring high energy efficiency.
The armature is connected to the valve via a shaft, and during operation of the solenoid actuator, the armature is attracted alternately by the upper and lower electromagnets to reciprocate vertically to open and close the valve. The armature attracted by the upper and lower electromagnets during the operation of the solenoid actuator is brought into abutment with the attracting surfaces of the cores of the electromagnets.
Therefore, the above conventional cores of the solenoid actuator suffer from the problem that impact of the abutment of the armature on each core during operation of the solenoid actuator can cause weld crack, thereby causing deformation and breakage of the laminates, which results in malfunction of the solenoid actuator. Further, laminates at opposite ends of the core, which are formed of the magnetically soft material, generate magnetic fields between magnetically soft components around the core and themselves during energization of the electromagnet, whereby part of energy of the electromagnet is lost.
It is an object of the invention to provide a core of a solenoid actuator, which is improved in durability, and at the same time ensures high energy efficiency of the solenoid actuator.
To attain the above object, the present invention provides a core of a solenoid actuator, for attracting an armature during operation of the solenoid actuator, comprising:
a plurality of core plates made of a magnetically soft material and stacked in a predetermined direction orthogonal to a direction of attracting the armature to form a laminated stack, for forming magnetic circuits between the armature and the core plates themselves during the operation of the solenoid actuator;
two core holders formed of a non-magnetic material and sandwiching the laminated stack of the plurality of core plates from opposite sides along the predetermined direction;
an insulator interposed between each adjacent two of the plurality of core plates, for insulating the each adjacent two core plates from each other; and
fixing means formed of a non-magnetic material and rigidly securing the plurality of core plates and the two core holders to each other to form a unitary assembly,
wherein the plurality of core plates are each formed with at least one projection projecting outward from a surface thereof and at least one recess each formed in a reverse side of each of the at least one projection, the at least one projection of one of the each adjacent two core plates being fitted in the at least one recess of another of the each adjacent two core plates such that the each adjacent two core plates are inhibited from relative movement with respect to each other in the direction of attracting the armature.
According to this core (including a yoke forming a magnetic circuit between the armature and the yoke itself) of a solenoid actuator, since the plurality of core plates simply stacked in one direction and the two core holders are secured to each other to form a unitary assembly, in a state of the plurality of core plates being sandwiched between the two core holders, the core can be made simpler in construction than the conventional ones, which contributes to reduction of manufacturing costs. Further, projections of one core plate are fitted in recesses of another core plate adjacent thereto such that the two adjacent core plates are inhibited from relative movement with respect to each other in the direction of attracting the armature, so that even if the plurality of core plates receive an impact caused by e.g. abutment of the armature on the core during operation of the solenoid actuator, the core plates cannot be displaced with respect to each other in a direction in which the armature moves, and hence it is possible to prevent breakage of each core plate, thereby enhance the durability of the same. Further, the projections and recesses can be used for positioning the plurality of core plates with respect to each other in assembling them into a laminated stack, which facilitates the assembly work of the laminated stack. Further, according to the core of the invention, since the plurality of core plates are stacked in the predetermined direction orthogonal to the direction of attracting the armature, in a state insulated from each other by the insulators, a magnetic circuit is formed between each core plate and the armature during the operation of the solenoid actuator, and at this time, an eddy current is generated in each core plate. However, since the core of the invention is formed by the plurality of core plates each of which is thinner than an ordinary solid core, the eddy current generated in each core plate disappears more promptly than in the solid core. Moreover, since the two core holders at the opposite ends of the core are formed of the non-magnetic material, magnetic fields are not readily generated between the core and magnetically soft components around the core during operation of the solenoid actuator, which contributes to reduction of energy loss. Thus, the core of the invention makes it possible to ensure high energy efficiency of the solenoid actuator.
Preferably, the laminated stack of the plurality of core plates is formed with at least one through hole extending therethrough in the predetermined direction, and the fixing means comprises at least one rod each extending through a corresponding one of the at least one through hole and fixed to the two core holders.
According to this preferred embodiment, since displacement of the core plates with respect to each other in the direction in which the armature moves is prevented not only by the fitting of the projections of each core plate in the recesses of its adjacent core plate, but also by at least one rod extending through the laminated stack of the plurality of core plates, it is possible to prevent breakage of each core plate more positively and further enhance the durability of the same.
Preferably, an upper face of the laminated stack of the plurality of core plates is lower than upper faces of the two core holders by a predetermined height.
According to this preferred embodiment, when the armature is attracted by the core of the solenoid actuator, the armature is brought into abutment with the two core holders alone, without being brought into contact with the plurality of the core plates, so that most of impact caused by the abutment of the armature on the core can be received by the core holders.
Preferably, each of the plurality of core plates has a whole surface thereof coated with an insulating film, and the insulator is formed by corresponding portions of the insulating films of the each adjacent two of the plurality of core plates.
According to this preferred embodiment, an eddy current generated in each core plate during operation of the solenoid actuator is inhibited by the insulating films from flowing to its adjacent core plates and disappears promptly within the thin core plate.
Further, the non-magnetic material forming the two core holders and the fixing means is an austenitic stainless steel having stiffness.
According to this preferred embodiment, since the core holders and the fixing means are each formed of an austenitic stainless steel having stiffness, they cannot be deformed even if they receive the impact caused by the abutment of the armature on the core.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.